A. Transcription termination at the end of the trp operon: In order to establish a system for characterizing the transcription termination signal at the end of the trp operon (ttrp), we have isolated a series of deletion strains in which the lac operon is brought close to but not fused to the trp operon. In these strains, ttrp is intact, but the lac promoter is, at least, partially deleted. Since the strains are thus lac negative, mutations which alter ttrp, or its functioning, will result in fusion of lac to the trp operon and a lac positive phenotype. The nature of ttrp can now be analyzed by selecting for lac positive revertants in such strains. B. Regulation of the lac operon by ppGpp: We have found conditions under which the expression of the lac operon is stimulated (30-70 fold) by ppGpp both in vivo and in vitro. This stimulation will be used as a handle on isolating mutants which alter this effect. Mutations which do this could either be in the lac promotor or in a protein factor (or RNA-polymerase) which mediates the effect. The mutants will be studied genetically and biochemically in order to understand the molecular mechanism of this control. C. Regulatory mutants affecting su3 gene expression: We have isolated fusions of the lac operon to the su3 (tyrosine tRNA) gene. We can now use the expression of the lac operon to monitor su3 gene expression. In addition, we can isolate mutants which alter su3 gene expression by selecting for effects on the lac operon. The mutations could either affect the su3 gene controlling element region (promoter, etc.) or other factors in the cell involved in regulating this gene. Such mutants will be used to help understand how stable RNA is regulated.